1. Field of the Invention
This invention relates to molecular beam deposition of layers of material and, more particularly, to effusion cells and methods for their use in the molecular beam epitaxial (MBE) growth of such materials.
2. Discussion of the Related Art
Molecular beam deposition of layers of material (e.g., semiconductors, metals, insulators, or superconductors) on a heated substrate in an ultra high vacuum is well known in the art. In particular, MBE is one of the principal techniques used in the semiconductor device industry to fabricate high quality, single crystal, semiconductor layers with thickness control on the order of a monolayer. In MBE a single crystal substrate or wafer is placed in a vacuum chamber where it is heated. Effusion cells loaded with source materials are used to generate beams of constituent atoms, which are directed at the substrate. The constituent atoms adsorb on the substrate surface and incorporate into the underlying crystal structure to form a semiconductor layer. Control is so good that the layer is literally formed one monolayer at a time.
Although the term molecular is used to describe the source material in this deposition process, those skilled in the art understand that the source material may be in elemental (or atomic) as well as compound (or molecular).
In the growth of Group III-V compound semiconductor layers, for example, the crucible of one effusion cell would contain a Group III metal (e.g., liquid Ga) and the crucible of another cell would contain a Group V material (e.g., elemental As, or less commonly polycrystalline GaAs). One persistent problem in MBE technology is that droplets of Ga, which form on the cooler tip end of the Ga crucible, become encapsulated with a thin surface layer of GaAs. It is believed that the GaAs-encapsulated Ga droplet can then explode causing small Ga deposits to be spattered onto the growth surface of the substrate. These adsorbed Ga spatters disrupt the crystal symmetry of the growing layer and hence are undesirable in high quality Group III-V compound semiconductor layers.